1. Field of the Invention
The invention relates generally to the field of loudspeakers employing a plurality of substantially rigid planar diaphragms, driven by cooperating coil and magnet units.
More specifically, the invention relates to a planar loudspeaker system having an elongated and substantially planar enclosure, configured to house tweeter, midrange, and a pair of woofer diaphragms in its median, longitudinal portion. A pair of quarter-wave woofer labyrinths extends throughout the remainder of the enclosure's volume. The labyrinths vent the woofer backwaves forwardly, toward the listener, through a pair of shared ports in the mid-frontal area of the enclosure.
The invention also relates to electro-magnetic drive units which apply a distributive driving force to the planar diaphragms, while presenting a distributive load to the drive amplifiers. Three versions or embodiments of the drive units are disclosed, each of which utilizes bar magnets, pole pieces, and thin encapsulated moving coils, all of elongated and symmetrical configuration, in accordance with the distributive drive and load design.
2. Description of the Prior Art
a. Planar Loudspeakers
An overview of planar loudspeaker designs is presented on pages 40-47 of the August, 1987, issue of Audio, Audio Publishing, New York, N.Y. While by no means complete, this article does present some of the varied historical and current approaches to constructing and driving, and housing planar diaphragms. However, none of these designs contemplates the particular loudspeaker system disclosed in the following application.
The applicant herein has previously described, in U.S. Pat. No. 4,385,210, an electro-acoustic planar transducer which shares at least some of the characteristics and goals of the loudspeaker system presented herein. Specifically, the transducer disclosed in the '210 Patent uses substantially rigid planar diaphragms, driven by an electro-magnetic drive system. The design taught in the '210 Patent also addresses the "backwave problem", in the particular context of a planar loudspeaker mounted directly upon a rearwardly positioned wall, or other planar surface.
Applicant's prior design, however, was not adapted for multiple transducer systems to be positioned immediately adjacent one another, as the laterally directed backwaves of adjacent systems would not be able to vent properly. Also, in some circumstances, the laterally vented backwaves can induce undesirable resonances in the rear wall upon which the speaker is designed for mounting. Lastly, the stationary coil and moving magnet drive disclosed in the '210 Patent presents such a low impedance to the amplifier, that even current designs for high fidelity amplifiers have considerable difficulty in driving the transducers properly. The planar loudspeaker design herein is directed toward solving each of these issues, while retaining the segmented, rigid planar diaphragm and electro-magnetic drive philosophy associated with the applicant's prior design.
b. Electro-Magnetic, Moving Coil Drive Systems
A summary of the construction and operation of a conventional moving coil/stationary magnet drive system for a direct radiator dynamic loudspeaker is shown on page 337, Section 9.3 of Music, Physics And Engineering by Harry F. Olson, Second Edition, Dover Publications, Inc., New York, N.Y. The typical electro-magnetic drive unit shown therein employs a single, conical shaped permanent magnet, a first cylindrical pole piece on the forward end of the magnet, and a second yoke-shaped pole piece, extending from the rear end of the magnet around to the forward portion of the first pole piece. A ring-shaped voice coil, attached to a cone diaphragm, is positioned within a slightly larger aperture of corresponding configuration, located between the adjacent ends of the pole pieces. When an electrical signal is applied to the voice coil, a force to drive the diaphragm is produced by the interaction between the electro-magnetic field and the magnetic flux lines flowing between the pole pieces.
Symmetry is lacking, both in the structure and in the dynamic response of this single magnet, dual pole drive system. It is evident that the first and second poles differ considerably in mass, size and configuration. The resultant flux field, which the coil intercepts and reacts with, is non-linear from front to rear, causing the dynamic response of the loudspeaker to be similarly affected.
This non-symmetrical operation is also inherent in alternative constructions, such as the single ring magnet, dual pole electro-magnetic speaker drive units of more contemporary design. This construction is shown in an article entitled "Rebuilding the AR-1", contained in the February, 1982 issue of SPEAKER BUILDER, Edward T. Dell, Jr., Peterborough, N. H. Again, owing to the differences in mass, size, and configuration of the pole pieces, the driven pistonic action of the moving coil is non-linear.
Another characteristic of both of the aforementioned drive units is their application of drive force through a relatively small implement, namely, the ring-shaped structure supporting and forming the moving coil. While such a structure may be well adapted for driving a cone-shaped speaker diaphragm, it is not particularly suited for driving a large, substantially rigid, planar diaphragm. The obvious expedient of employing a plurality of such moving coil drive units would add considerable weight to the planar diaphragm, and would detract from its ability to respond properly to transients. Furthermore, the application of driving force to the diaphragm would still be made through a relatively small number of pressure points, increasing the likelihood of diaphragm flexure under heavy drive conditions.
Applicant's electro-magnetic drive system addresses the above-mentioned problems of non-symmetrical operation and point application of force in a moving coil/stationary magnet construction. And, the present electro-magnetic drive units are ideally suited to actuate the lightweight, substantially rigid, planar diaphragms of the planar loudspeaker system herein.